Epoxide resins are often used as reaction resins to coat or bond electronic components and modules. These epoxide resins are then hardened. Reaction resin mixtures having an epoxide resin base can be activated to harden--when the right accelerators or initiators are present--both thermally as well as by means of UV radiation. Known initiators for hardening in accordance with a cationic mechanism are sulfonium salts, which--depending upon the chemical constitution--form the initiating species through thermal or photochemical activation.
Sulfonium salts that are capable of being thermally activated are described, for example, in J. Polym. Sci. Polym. Chem. Ed., vol. 29 (1991), pp. 1535-1543. Benzylthiolanium salts having non-nucleophilic anions, which are suited for the cationic polymerization, such as PF.sup.-.sub.6, SbF.sup.-.sub.6 and BF.sup.-.sub.4, are of particular technical interest (c.f. JP-Unexamined Patent Application 58-37003 or Chemical Abstracts, vol. 99 (1983), no 141034v and J. Appl. Polym. Sci., vol. 32 (1986), pp. 5727-5732). In many cationically polymerizable reaction resin mixtures, these compounds demonstrate adequate latency at room temperature, i.e., formulations containing benzylthiolanium salts as hardening initiators are stable in storage. The European Unexamined Patent Application 0 379 464 describes araliphatic sulfonium salts, such as diethylbenzyl-sulfoniumhexafluoro antimonate, which are likewise supposedly suited as latent hardening initiators. However, the mentioned sulfonium-salt compounds are only suited for the thermally activated hardening of cationically polymerizable compounds.
Furthermore, it is known that the UV hardening of cationically polymerizable compounds can be initiated by triarylsulfonium salts (c.f., e.g., U.S. Pat. No. 4,058,401 and U.S. Pat. No. 4,138,255). An important characteristic of triarylsulfonium salts is their high thermal stability (c.f.: Adv. Polym. Sci., vol. 62 (1984), pp. 1-48). This means, however, that triarylsulfonium salts are unsuited for the thermal hardening of cationically polymerizable compounds.
For the combined thermal and UV hardening of cationically polymerizable compounds, which is technically desirable, sulfonium salts are known, which are able to be activated thermally and photochemically. These properties are demonstrated by sulfonium salts that are heterocyclic, aryl-substituted or anellated with an aryl system (c.f. WO 90/11303), as well as by phenylbenzylalkyl sulfonium salts (c.f. EP Unexamined Patent Application 0 331 496) and naphthalene sulfonium salts (c.f. JP Unexamined Patent Application 63-152619 or Chemical Abstracts, vol. 109 (1988), no 232200n). However, the technical applicability of many of these compounds is clearly restricted by a number of disadvantages. These include, in particular, the often too low solubility in technically practical formulations and the often too low UV absorption above 315 nm. The result is that the UV-A radiation from the mercury-radiation emitter used principally for the UV hardening is not absorbed effectively. However, a hardening initiation with UV-A radiation is especially desired in the case of thick layers, since it achieves a greater penetration depth and allows fewer scattering losses to occur. Moreover, due to occupational safety and health considerations, a hardening with UV-A radiation can also be desirable, since--compared to UV-B and UV-C radiation--this radiation clearly has less of a photobiological effect.
From a standpoint of technology and economics, it is an important task, for example in the field of electronics, to harden a cationically polymerizable reaction resin mixture, that is stable in storage as a single-component system, by means of UV radiation and heating. Thus, a combined hardening in the coating (lacquering, covering, sheathing) and bonding of electronic components and modules is always necessary when there are regions that are shaded from light, where the reaction resin cannot be reached by the UV radiation, and which, therefore, must be thermally hardened. A combined hardening is also necessary, when, for technical reasons, the reaction resin mixture contains additives, such as fillers, pigments, and dyes, or also resinous constituents, which absorb or scatter the UV radiation in the upper layers so heavily, that there is no longer sufficient UV intensity in the deeper layers. This is the case, for example, when unhoused ICs are covered locally on hybrid circuits by means of a reaction resin drop.